1. Field of the Invention
This invention relates to an improvement on an electromagnetic drive device which operates a shutter blade or the like of a photographic camera with driving force obtained from a driving electric current flow to a coil provided on a rotor arranged within a magnetic field.
2. Description of the Prior Art
Many camera shutters have been contrived of late to be operated by electromagnetic drive devices which better permit structural simplification than the conventional mechanical shutters. In the cameras of this type having the electromagnetic drive device, arrangement to have the electromagnetic drive device also control the action of a mechanism participating in an exposure, such as an automatic focusing device or the like, in addition to the shutter driving action thereof dispenses with a magnet or the like which is conventionally used for locking such an exposure participating mechanism. This arrangement, therefore, is advantageous in terms of space and cost for cameras which are required to be as compact as possible. However, there has been a shortcoming in the prior art arrangement of the above stated type. The shortcoming resides in that, with the electromagnetic drive device arranged to perform combined functions of controlling an automatic focusing device and controlling the operation of shutter blade members or the like, the shutter blade members might be operated to accidentally effect an exposure when the automatic focusing device is controlled by the electromagnetic drive device.
To solve the above stated problem, the present inventor has previously proposed a method as disclosed in a U.S. patent application Ser. No. 381,785, filed by the same applicant as the present application. This prior art method is as described below with reference to FIG. 7 of the accompanying drawings:
When a shutter release button of a camera is depressed, a distance measuring mechanism which is not shown operates to detect a distance to an object to be photographed. After that, a control circuit which is not shown causes an electric current flow to a coil 101. Then, there is produced an electromagnetic driving force, which causes a rotor 102 to rotate clockwise while charging a spring 103. A driving pin 102b pushes an acting part 111d of a clamping lever 111. Meanwhile, this rotation also disengages a pushing part 104b of a sector ring 104 from a protrudent part 102a of the rotor 102. At this time, however, the sector ring 104 is prevented from rotating by an arm part 104c thereof as the arm part engages a notch 128a provided in a sector ring lever 128. Therefore, shutter blades 107 remain stationary.
With the acting part 111d of the clamping lever 111 pushed by the driving pin 102b, the clamping lever 111 rotates counterclockwise to disengage a pawl part 114a of an automatic focusing (hereinafter will be called AF) start lever 114 from a first clamping part 111a of the lever 111 and to position a protrudent part 111c thereof within an operating region of a transmitting lever 117. Then, a start pawl 119 is caused by the urging force of a spring 115 to rotate counterclockwise together with the AF start lever 114. With the pawl 119 thus rotated, the claw part 119a thereof disengages from a protrudent part 122a of a set plate 122 to allow the set plate 122 to be moved by the urging force of a spring 120 downward as viewed on the drawing. This downward motion of the set plate 122 causes a photo-taking lens 125 to move inward along a cam 122d of the set plate 122. Meanwhile, the motion of the set plate 122 also causes an arm part 122c of the set plate 122 to let go its hold of the transmission lever 117. Accordingly, the urging force of the spring 115 is transmitted through the transmission lever 117 to cause an AF stop lever 126 to rotate clockwise. However, the rotation of the lever 126 comes to a stop at a position where it comes to engage the protrudent part 111c of the clamping lever 111. After that, when the photo-taking lens 125 is moved by the movement of the set plate 122 to a position corresponding to the distance to the object to be photographed, the power supply to the coil 101 from the control circuit is cut off. Therefore, when the photo-taking lens 125 reaches the infocus position, the spring 103 causes the rotor 102 to rotate counterclockwise. The counterclockwise rotation of the rotor 102 causes the clamping lever 111 to rotate clockwise and the protrudent part 111c of the clamping lever 111 disengages from the transmission lever 117. Therefore, the urging force of the spring 115 causes the transmission lever 117 and the AF stop lever 126 to rotate until the pawl part 126a of the AF stop lever 126 engages a toothed part 122e of the set plate 122 to bring the movement of the set plate 122 to a stop. Meanwhile, the transmission lever 117 pushes an end part 128c of the sector ring lever 128 to cause the lever 128 to rotate against the urging force of a spring 130. The rotation of the sector ring lever 128 causes an adjustment lever 131 to rotate clockwise on a fulcrum 132. This rotation of the adjustment lever 131 increases the urging force of the spring 103 while the arm part 104c of the sector ring 104 is disengaged from the notch part 128a of the sector ring lever 128. With this, an automatic focusing action (hereinafter will be called an AF action for short) comes to an end.
Upon completion of the AF action, the control circuit again causes a power supply to the coil 101 to cause the rotor 102 to rotate clockwise. At this time, however, since the pawl part 114a of the AF start lever 114 is in engagement with a second clamping part 111b of the clamping lever 111, the acting part 111d of the clamping lever 111 has been withdrawn from the separating region of the driving pin 102b. Therefore, the rotation of the rotor 102 is never affected by the clamping lever 111. Meanwhile, since the sector ring 104 is not in engagement with the sector ring lever 128 at this time, the urging force of the spring 105 causes the sector ring 104 to follow the clockwise rotation of the rotor 102. Accordingly, the shutter blade 107 rotates on a fulcrum 108 to begin an exposure. Then, an exposure control mechanism which is not shown begins to count the quantity of incident light. When a predetermined quantity of the incident light has been detected by the exposure control mechanism, the control circuit cuts off the power supply to the coil 101. Since the urging force of the spring 103 is arranged to be sufficiently larger than that of the spring 105, the pushing part 104b of the sector ring 104 receives a strong returning force in the counterclockwise direction from the protrudent part 102a of the rotor 102. Therefore, when the power supply to the coil 101 is cut off, the moving direction of the sector ring 104 is swiftly reversed to close the shutter blade 107. Such arrangement permits control over a high speed shutter time.
Upon completion of the above described photographing operation, a charging mechanism which is not shown moves the set plate 122 upward as viewed on the drawing to charge each mechanism of the camera and thus to bring the whole camera back to the state as shown in FIG. 7.
However, in accordance with the above described prior art arrangement, while the automatic focusing action is controlled by the driving action of the electromagnetic drive device, the shutter blade must be kept in a closed state against a spring force to prevent it from being opened by the driving force during the automatic focusing action. Then, in a camera which is not allowed to have a large power source for the sake of compactness, the driving force of the electromagnetic drive device becomes so weak that it is hardly possible to keep the shutter blade closed against the spring force over a long period of time.
Further, the prior art arrangement described above has operating members arranged out of harmony resulting in insufficient precision of their interlocking relation and also has presented a problem with regard to reduction in size.